Conductive transparent layers are of great significance for applications in electronics and optoelectronics, for example in displays, electronic paper, solar cells, touch panels and as an electrode. To date, owing to good electrical conductivity and established industrial implementation, principally tin-doped indium oxide (ITO) and in some cases also fluorine-doped SnO2 (FTO) have been used, which are typically applied to the substrates by means of costly and inconvenient application technology (sputtering). Another great disadvantage is the high costs for indium.
In the coating of polymeric substrates, the adhesion of the layers is additionally critical. Layers of transparent conductive oxides (TCOs) applied to substrates by chemical vapor deposition (CVD) are generally very brittle and therefore become detached very easily from thin substrates, for example polymer or glass. TCO layers produced in this way also have a marked surface roughness, which is disadvantageous especially in components with several layers and in the case of layer thicknesses in the region of 100 nm or less (for example OLEDs).
The production of transparent conductive oxides (TCOs) by means of sol-gel processes and generation of corresponding layers has been proposed as a possible solution to the above-described problems. The mesoporous structure required is generated in the prior art typically by templating, using structure-forming components, for example nonionic surfactants, to control or influence the mesostructure.
However, a disadvantage in the known processes is that the crystallinity, which is a prerequisite for high conductivities, of the layers applied to substrates, for example by dip-coating, has to be increased by calcining at high temperature, which frequently leads to crack formation and detachment of the films from the substrates.
JP 2005-060160 A describes the production of mesoporous films proceeding from metal halides by templating by means of polyoxyethylene stearyl ether and subsequent aging in a steam atmosphere below 100° C. However, a disadvantage is the complicated and time-consuming process and especially the very low crystallinity and conductivity, and also the stability of the mesostructure of the TCO thus obtainable, which is insufficient at high temperatures.
WO document 99/37705 discloses that mesoscopically ordered oxide-block copolymer composites and mesoporous metal oxide films can be obtained by using amphiphilic block copolymers in aqueous medium, which function as structuring agents by self-assembly. The block copolymers used are alkylene oxide block copolymers and EO-PO-EO triblock copolymers. The pore sizes thus obtained are up to 14 nm. The oxides described include TiO2, ZrO2, SiO2, Al2O3, SnO2. Conductive transparent oxides are not mentioned. When said alkylene oxide block copolymers are used, a destruction of the mesostructure during the thermal treatment, if at all, can be prevented only by complicated temperature programs. An additional disadvantage is the low crystallinity of the nonstoichiometric oxides. The process of WO 99/37705 additionally proceeds in the presence of water and does not lead to thin layers with homogeneous layer thickness for transition metal oxides.
The publication of Brezesinski et al., Advanced Functional Materials 2006, 16, 1433-1440 describes the use of poly(ethylene-co-butylene)-block-poly(ethylene oxide) as a template (in the context of so-called EISA, evaporation-induced self-assembly) for the formation of the mesostructure in the production of mesoporous highly crystalline thin layers of SnO2.
Fattakhova-Rohlfing et al., Advanced Materials 2006, 18, 2980-2983 describe the preparation of transparent indium tin oxide (ITO) by means of EISA in conjunction with poly(ethylene-co-butylene)-block-poly(ethylene oxide) as a structuring agent in a sol-gel process. However, a disadvantage is the limited dissolution behavior of poly(ethylene-co-butylene)-block-poly(ethylene oxide), which requires the presence of high amounts of tetrahydrofuran (THF) and can lead to incompatibility with regard to the solubility of the constituents of the reacting compounds, especially in complex mixtures. The processes described in the publications cited are unsuitable for the preparation of numerous TCOs, especially of antimony-doped tin oxide. In addition, the low mean pore size of the TCOs thus obtainable leads to a reduced stability during crystallization at high temperatures.
The use of block copolymers comprising a polyethylene oxide block and an isobutylene oxide block for templating in the preparation of mesostructured silicon dioxide and titanium dioxide is known from the publication of Groenewolt et al., Advanced Materials 2005, 17, 1158-1162. This describes the use of PIB85-PEO79 for preparing mesoporous silicon dioxide by a sol-gel process proceeding from TMOS, and mesoporous TiO2 proceeding from TiCl4. The diblock copolymer has a structure-forming function as a result of self-assembly. However, the publication does not disclose the preparation of transparent conductive oxides.